The invention is directed to a device and corresponding method for testing materials for structural weakness. In the preferred embodiment, the invention is directed to a device and a method for testing ball and/or roller bearings to determine their tendency to experience fretting.
Ball bearings and roller bearings are components that are used in vast array of machine tools and high-performance instruments. Bearings are found in an enormous variety of moving components, ranging in magnitude from heavy earth-moving equipment to delicate instruments such as disc drives, video tape recorders, and high-precision machine instruments. In short, the rolling contact bearing (i.e., ball bearings, roller bearings, and the like) is a fundamental element of machinery that, in many instances, determines the overall performance characteristics of the machine. If a particular bearing breaks or seizes, for example, not only is the individual segment or module wherein the bearing lies compromised, but the entire instrument is likely to cease functioning properly. Further, due to the very tight tolerances inherent in their function, if one bearing begins to break down or deform, it will generate intense friction and result in overheating which will degrade the race or track in which the bearing rides. This in turn will cause other bearings with which it is housed to become deformed, leading to a domino effect that rapidly affects the entire bearing assembly.
It follows from the above paragraph that the physical specifications of bearings are extremely important in manufacturing and maintaining precision instruments and machinery that have moving parts. In order to construct bearings and their races, and to keep them within precise tolerances over a defined period of time, a number of parameters must be monitored. These parameters include the specific dimensions of each bearing and its race, the hardness of those components, and their ability to withstand impact, resist corrosion, and resist distortion under load. Although all bearings become unserviceable over time due to wear, they may also become unserviceable because of seizing, breakage, undue wear or premature aging, false brinelling, flaking, and corrosion. Bearing failure can also be the result of rolling fatigue, incorrect selection, improper handling and/or improper maintenance, excessive load, poor shaft or housing accuracy, peeling, spalling, chipping, cracking, rust, corrosion, and fretting.
Many methods have been described to increase the life of bearings and decrease their incidence of breakdown. These include special alloys from which to fabricate the bearings (see, for example, U.S. Pat. Nos. 6,358,333; 6,422,756; and 6,315,455, all to Tanaka); use of ceramic bearings (see U.S. Pat. No. 6,416,228); use of silicon hybrids (see, for example, NSK Robust Series), as well as specific designs for the bearing structure itself (see U.S. Pat. Nos. 5,700,093 to Hiramatsu and 5,803,614 to Tsuji). Clearly, any method to optimize bearing performance must rely on tests to measure the physical characteristics of the bearing itself, both before and after the bearing has been put to its prescribed use.
Conventional methods to test bearing performance include simple performance verification tests wherein a prototype of the bearing is put through high pressure, high speed heavy duty use, etc., to verify bearing loss, seizure, and service life. (See, for example, Mitsubishi Heavy Industries, Ltd., Technical Rev. 39:26-30 (2002).) Other conventional means of testing bearing endurance include seizure tests wherein machinery is operated using the prototype bearing and the oil supply is curtailed while the machine continues to run. By measuring the oil flow and temperature, the response time required to save the system from breakdown can be determined. See SensIT Newsletter #3, June, 2001. Other means to test bearing breakdown include vibration testing to determine fretting resistance and impact resistance. All of these tests are implemented on bearing assemblies and the test conditions in local contact areas cannot be controlled precisely. Therefore, the failure modes of the bearings can only be investigated statistically.
Methods used to make conventional bearing assessments rely on mechanical mechanisms. Currently, conventional actuators, such as step motors, servo motors, and hydraulic or pneumatic actuators are used to deliver force or pressure in a test environment. A particular disadvantage when using conventional actuators is their poor dynamic response to a quick force change, especially under a heavy load.
More recently, electronic equipment and computer chips have been making use of piezoelectric devices as switches, thereby obviating the need for mechanical switches with their size and inherent error. The piezoelectric effect was first discovered when a pressure was applied to a quartz crystal and an electric charge in the crystal was created. It was later found that by applying an electric charge to the crystal the material would deform in shape in a standard degree. The first use of the piezoelectric effect was in ultrasonic submarine detectors developed during World War I. It was later found that barium titanate ceramics could be made which exhibited the piezoelectric phenomenon.
Because the tolerances of bearings are so precise and their proper action crucial to their operation, there is a need for the development of bearing test mechanisms that can apply force in a way that is controlled to deliver similar magnitude to a much more discrete area and measure the results on a nanoscale range. Further, because bearings are inherently moving parts, they are particularly subject to dynamic stress that can lead to fretting of the bearing.
Fretting is a form of adhesive wear; it occurs as the result of small scale oscillatory movements between the bearing and its housing. Typically fretting appears as highly polished regions on the bearing surface or as pockmarks on the bearing. Fretting is also accompanied by evidence of material movement between the housing and bearing back. Fretting occurs when there is unwanted relative movement between the bearing and housing. This movement can arise from any number of sources, such as an oversized housing, dirt or burrs on mating faces of the housing, insufficient bolt torque, deformation of the bearing under load, etc. Another cause is the flexibility of the bearing assembly itself. If the material from which the bearing and/or bearing race is assembled has inappropriate stiffness, the entire assembly may flex sufficiently under dynamic load to cause a relaxation of the radial interface and allow unwanted relative movement between the bearing and the race.
Fretting can eventually lead to overheating of the bearing material due to poor heat dissipation between the bearing back and the housing. This, in turn, leads ultimately to failure of the bearing entirely or generation of excessive heat that causes other components to fail.
The invention described herein was designed to investigate, measure, and otherwise quantify and/or qualify the fretting resistance of a bearing material. The invention is capable of controlling the load placed on a hearing while simultaneously subjecting the bearing to precisely controlled oscillatory movement. The invention thus yields superior dynamic results, and reduces the time-of-testing cycle significantly.
A first embodiment of the invention is directed to a device for testing the fretting of a bearing. In this embodiment, the device is dimensioned and configured so that a first object (e.g., a ball bearing) is urged against and dynamically translated, rotated, or otherwise urged against the surface of a second object (e.g., a flat plate of bearing housing material). In this embodiment of the invention, the device comprises a force scanner which comprises a first piezoelectric actuator. A workpiece holder is also optionally included, the workpiece holder being dimensioned and configured to hold a pre-selected workpiece securely and releasibly. (In the preferred embodiment, the workpiece holder is dimensioned and configured to hold a ball or roller bearing.) The force scanner is specifically dimensioned and configured to deliver a predetermined force to the bearing (i.e., the first object) in a first axis of motion (e.g., in the vertical direction). The predetermined force is generated by piezoelectric movement of the fist piezoelectric actuator.
The device further comprises a motion scanner. The motion scanner comprises a second piezoelectric actuator. The motion scanner is specifically dimensioned and configured to translate the second object in a second axis of motion, wherein the second axis of motion is different from the first axis of motion. In the preferred embodiment, the second axis of motion is substantially orthogonal to the first axis of motion. The translation of the second object is caused by piezoelectric movement of the second piezoelectric actuator. The first embodiment further comprises a first sensor operationally connected to the force scanner. The first sensor is capable of measuring the force delivered to the bearing by the force scanner and provides a feedback signal to a controller for a closed loop control (force control). A second sensor is operationally connected to the motion scanner. The second sensor is dimensioned and configured to measure the motion delivered to the second object by the motion scanner and to provide a feedback signal to the controller for a closed loop control (motion control).
As used herein, the term xe2x80x9coperationally connectedxe2x80x9d designates that the recited elements are xe2x80x9cconnectedxe2x80x9d in a functional sense, meaning that, for example, the first sensor recited in the previous paragraph will function as a sensor because it is functionally connected, directly or indirectly, to the force scanner. Thus, elements that are xe2x80x9coperationally connectedxe2x80x9dare not necessarily linked directly to one another, but may be separated by intervening elements that do not interfere with the operational relationship of the xe2x80x9coperationally connectedxe2x80x9d elements.
A second embodiment of the invention is directed to a device for testing fretting of a bearing. In the second embodiment, the device comprises a force scanner including a first piezoelectric actuator, a first flexure stage, and a first cantilever. The first cantilever is interposed between the workpiece holder and the first piezoelectric actuator and is operationally connected to both the workpiece holder and the first piezoelectric actuator. The first cantilever is dimensioned and configured to amplify the motion generated by the first piezoelectric actuator. The amplification is to compensate for the force lost: (1) due to the compliance in the contact between the first and the second objects; and (2) due to wearing and the stiffness of the workpiece and the contact. The first flexure stage is dimensioned and configured to move in a first axis of motion and to resist motion in all other axes of motion. The force scanner as a whole is dimensioned and configured to deliver a predetermined force to the first object in the first axis of motion, the predetermined force being generated by piezoelectric movement of the first piezoelectric actuator and amplified by the first cantilever.
The second embodiment also includes a motion scanner that comprises a second piezoelectric actuator, a second flexure stage, and a second cantilever. The second cantilever is interposed between the second flexure stage and the second piezoelectric actuator and is operationally connected to both the second flexure stage and the second piezoelectric actuator. The second flexure stage is dimensioned and configured to move in a second axis of motion that is different from the firs axis of motion. In the preferred embodiment, the second axis of motion is substantially orthogonal to the first axis of motion. The second flexure stage is dimensioned and configured to resist motion in all other axes of motion. The second flexure stage is also dimensioned and configured to amplify the motion generated by the second piezoelectric actuator, to thereby achieve a longer scanning range in the second axis of motion. The motion scanner is dimensioned and configured to translate the second object in the second axis of motion, and wherein translation of the second object is caused by piezoelectric movement of the second piezoelectric actuator.
A first sensor is operationally connected to the force scanner. The first sensor is capable of measuring the force delivered to the object by the force scanner in the first axis of motion. A second sensor is operationally connected to the motion scanner. The second sensor is capable of measuring movement of the motion scanner in the second axis of motion. The outputs of both sensors are used to provide feedback for closed-loop control to achieve accuracy in the force and motion applied to the workpiece being tested. The closed loop control also serves to maximize accuracy in the phase matching between the applied force and the applied oscillating motion required by the testing.
The second embodiment of the invention further includes a closed-loop controller operationally connected to the first and second piezoelectric actuators. The controller is dimensioned and configured to independently control piezoelectric movement of the first and second piezoelectric actuators through a driver. The controller can be driven by a signal generator to control force applied by the force scanner, and translation of the motion scanner.
In this fashion, the system generates a precisely controlled, desired, and pre-determined contact force, friction force, and friction speed between the objects being tested. The force applied to the first object (i.e., the workpiece) in the first axis of motion is supplied by the force scanner. Movement of the motion scanner in the second axis of motion allows the device to produce relative motion and friction between the first object and the second object. The combination of the force and the motion scanners make it possible to simulate a wide variety of stress and strain conditions on the surfaces of the objects being tested. Thus, the invention enables objects to be tested for fretting (and other types of physical degradation and wear) in response to a known applied force, sliding velocity, distance, etc.
It is therefore an object of the present invention to provide a device and a corresponding method which can study and measure the effect of stresses, strains, and dynamic frictions on individual bearings and races. In a preferred embodiment of the invention, a Z-motion stage is designed to produce an accurately controlled contact force on the workpiece being tested. The friction force generated varies with the ambient operational conditions, such as the friction coefficient of the coupling materials, the friction velocity, the temperature, the lubricant, etc. The contact force is measured so that when the piezoelectric actuator of the Z-motion stage expands (i.e., in the first axis of motion), the first cantilever tilts downward, pivoted by a notch flexure at one end, and urges a first workpiece object (e.g., a bearing) against a second workpiece object (e.g., a race) on an X-motion stage. The reactive force on the first workpiece object is transferred to the force sensor mounted at the back of the workpiece holder of the Z-motion stage.
Note, however, that the friction force between the bearing and the race being tested will generate a bending moment on the first cantilever and be coupled into the force sensor due to the position offset between the friction force and force sensor. Unless accounted for, this bending moment will degrade the accuracy of the force control.
Thus, in order to decouple the crosstalk of friction force, and thereby improve the accuracy of the present invention, a flexure decoupling mechanism (i.e., means for decoupling frictional crosstalk) may optionally be interposed between the workpiece holder and the force sensor. The decoupling mechanism is dimensioned and configured to have a very low stiffness in the first axis of motion (to thereby maintain the sensitivity of the force sensor) and an extremely high stiffness in all other axes to decouple all the forces and moments except the force applied in the first axis of motion.
The force applied by the Z-motion stage (in the first axis of motion) is detected by a strain gauge sensor or any other suitable means for detecting a force applied to an object. The movement of the X-motion stage (in the second axis of motion) is detected by a capacitance sensor or any other suitable means for detecting motion. The output signals of the sensors are then amplified via suitable amplification circuitry and displayed using any means for display now known or developed in the future, such as an analog or digital meter, oscilloscope, light-emitting diode display, liquid crystal display, cathode ray tube display, and the like.
The primary advantage of the present invention is that it allows bearings, races, and other workpieces subject to dynamic stress and strain to be tested quickly and confidently, with unparalleled precision and accuracy. Moreover, it allows the workpieces to be tested systematically, in isolation, and under test conditions that can be completely controlled. Thus, using the present invention, bearings, races, and other workpieces can be tested under precisely known applied forces, at known temperatures, with well-controlled force-motion patterns, etc. The novel design of the flexure platform and the associated piezoelectric controls and sensor electronics enables fretting tests that were previously difficult or impossible to be conducted individually, to be conducted quickly, reproducibly, and with unprecedented control. This greatly improves the selection of appropriate bearing materials for any given purpose. As a result, the operational life span of the bearing assembly (or other friction-bearing surface), once placed into service, is greatly extended.